While virtualization only introduces a negligible overhead on machines with few cores, this is not the case when the number of cores increases. We can find such computers with tens of cores in todays data centers dedicated to the cloud computing, a resource management model which relies on virtualization. These large multicore machines have a complex architecture, called Non Uniform Memory Access (NUMA). Achieving high performance on a NUMA architecture requires to wisely place application threads on the appropriate cores and application data in the appropriate memory bank. In this thesis, we show how virtualization techniques modify the applications behavior by preventing them to efficiently place their data in memory. We show that the data misplacement leads to a serious performance degradation, up to 700%. Additionally, we suggest a method which allows the Xen hypervisor to efficiently virtualize NUMA architectures by implementing a set of generic memory placement policies. With an evaluation over a set of 29 applications on a 48-cores machine, we show that the NUMA policies can multiply the performance of 9 applications by more than 2 and decrease the virtualization overhead below 50% for 23 of them.